Variable frequency cavity resonator



July 31, 1951 E. G. MARTIN v VARIABLE FREQUENCY CAViTY RESONATOR Filed April 24, 1945 FIGB III II 4/ INVENTOR.

EDWARD G. MARTIN A rroRA/kr Patented July 31, 1951 UNITED STAT NT OFFICE insurance FREQUENCY osvrrr arsena'roa (01. i is-44) 7 Claims. ,5

This invention relates in general to cavity resonators and more particularly to cavity resonators of the type which can be adjusted to resonate at various frequencies.

In certain applications of ultra high frequency oscillators a resonant cavity equivalent to a parallel resonant LC circuit is used as the frequency determining element of the oscillator circuit.

The advantages of possessing control over the frequency of oscillation by means of a tuning method are obvious. Heretofore, it has been cus tomary in the art to alter the resonant frequency of a cavity, which is generally cylindrical in form and has a diameter appreciably greater than its depth, by inserting one or more screws or other projections into the cavity through a side wall. This effectively changes a dimension of the cavity in proportion to the depth of penetration of the screw or screws. Since the resonant-frequency of such a cavity is determined by its physical dimensions, the desired frequency alteration may be thus produced.

Difficulties with this tuning method lie in the fact that the cavity dimensions do not change symmetrically and precise calibration is difficult.

Accordingly, it is one object of the present invention to provide novel means of reducing the cavity size in a cavity resonator in a symmetrical fashion, and especially to enable variation of the radial dimensions of a generally circular resonating chamber.

Another object of the invention is to provide a. tunable cavity resonator susceptible to precise calibration.

Still another object is to provide a resonant cavity which may be tuned by a single control.

7 A preferred cross-sectional shapefor a resonant cavity is circular. The difiiculties inherent in continuously changing the radial dimension of a cylindrical cavity while preserving its cross" sectional symmetry are obvious. Symmetry can be maintained by making the depth dimension variable, but it has been found that such a tuning method is highly sensitive withresultant instability and difficulty of adjustment.

Therefore a hexagonal cross-section has been chosen as an approximation to a circular shape, the use of which combines the advantages of diametral tuning variation with the merits of continuous frequency control.

The precise method by which these aims are accomplished will be better understood upon reference to the following specification, claims, and drawings, in which: 7

Fig. 1 shows a form of symmetrically contractto effect simultaneous movement of the cavity walls its attached and connected parts being omitted; and

Fig. 3 is a cross section on the line 3--3 of Fig. 1, greatly enlarged.

Referring now to Fig. 1, a retaining ring 4 encloses a circular series of six approximately triangular block-like members 5 symmetrically arranged within the ring and hereinafter referred to simply as blocks, constructed of a conducting material. Each block is identical with the other five and they are so placed that a base face of each bounds one side of a symmetrical hexagonal volume. A respective pin I 4 is inserted slidably into each block, in a hole i3 which is of sufficient depth to allow further entry of the pin during contraction of the size of the cavity. The other end of pin it is secured in an extensicn or shoulder 18 on the next adjacent block, and a helical compression spring I5 is wound around pin it to insure that the blocks will return to their original positions after removal of a compressing force. A third hole I6 is drilled in each block to take a guide pin ll, one end of which projects through ring l, having an enlarged screw head set in the ring. Pin I1 is thus fixed and acts as a guide for block 5. A roller-carrying studiilis mounted rigidly in each bl'ockprojecting above the face of the block.

While the perimetral walls of the chamber are formed by substantially triangular solid blocks in this instance, the essential is that the body,

of each of these elements or equivalents have aninner wall surface which is normal to a radius from the center of the chamber at the middle of that face of the wall which is exposed. in the chamber, and that these elements be so shaped and mounted that they may be moved toward the center or retracted while following closely next adjacent faces of the block elements on each side. For this purpose the inner side of the triangle defining the general form of the block may be regarded as the base. The right hand side viewed from the center in Figure 1 extends straight from near the base to the inner face ofthe ring 4, a base truncation or fiat 24 being formed which is at right angles to the base face, the hole i3 opening through this fiat and extending into the block parallel to the base, while the spring I5 bears against the flat around the hole. The opposite or left hand side of the block,

' viewed as suggested, extends straight from an distance of the inner face of the ring 4, where the lateral extension or shoulder 18 is formed on the block, having an inner face at right angles to the left side of the block and parallel with the face or fiat 26 at the right side of the next block to the left. In this extension the rod i4 is fixed and the outer end of the spring [6 expands thereagainst so as to tend to separate the shoulder 18 and the face 24, with the result that the springs tend to keep all the blocks at the outer limits of their movements, determined by engagement of the studs against the outer sides of the slots 23.

The apex of the block is thus broadened, and the guide rod i! extends therethrough in the hole 16 parallel to the right hand side of the block. The slots 23 cross geometrical lateral projections of the axes of the guide pins I! so that rotation of the plate 22 develops a force resultant from the camming of the studs which is effective along paths defined by the guide pins.

By this construction the inner or base face of each block constitutes a wall element or bounding member of the cavity between the plates 33 and 38, and by reason of the guiding support of the pins H, as the blocks move inward the one at the right as viewed from the center of Figure 1, moves out of the path of the one to the left, allowing similar movement of the latter, throughout the series, so that a virtually continuous solid rigid wall is formed around the chamber throughout constriction and expansion of its size within the limits of the ring 4. In these movements, the left hand end of the inner or base face of each block slides along the base face of the next adjacent block, and its other end receives the left end of the next block to the right similarly.

It should also be noted that loose fit or movement of the block members 5, as might be due to wear or allowances for free movement or otherwise provided, would permit slight pivotal or rotary movement of the blocks around axes normal to the plates 33 and 38 so that the sharp angles of the blocks shown, at the left ends of their chamber faces might tend to separate from the base faces of the next adjacent blocks and leave gaps at the angles of the polygon figure of the chamber. The springs 15 will exert a force on the blocks tangent to such axes tending to force the pointed angles against the base faces of the next blocks and so preserve a good closed wall figure.

A cam plate 22 shown in Fig. 2 fits closely over the assembly shown in Fig. 1. It has a number of eccentric cam slots 23 extending at an angle to the directions of respective guide pins ll, each receiving the roller of one of the studs 20. A central opening 39 is provided in the plate 22 to allow plate 22 to fit around and revolvably on the hub 38' of the cavity body structure, the latter being spaced around an oscillator tube 34 with which this type of cavity finds use, the tube in this instance being contained concentrically in the hub, as in Fig. 3.

Fig. 3 shows a method of assembling the components shown in Figs. 1 and 2 to perform as a tuning element in a velocity-modulated type oscillator. A plate 33 of conducting material (or at least a proper reflective material) is placed around the neck of a radio oscillator tube 34 to act as a bottom wall for the tuning cavity. To permit extraction of energy from the cavity a coaxial line 35 is attached to plate 33. Inner conductor 36 of coaxial line 35 extends through a port 31 into the cavity to act as a polar coupling means between the cavity and the coaxial line, the other side of the circuit comprising the outer conductor of the cable. The circuit utilizing the tube 34, the elements of the tube, and the connections with the cable 35 may be conventional, and therefore are not illustrated. The structure comprising retaining ring 4 and blocks 5 is then placed so that the blocks rest on plate 22 which is large enough to cover the entire area enclosed by the ring 4. A collar plate 38 is set in a relieved part of the under face of the plate 22. It is of the same diameter as plate 33, lapping the blocks at their inner parts and forming the top wall of the cavity and plate 22 fits over the other three components to complete the assembly. The manner in which the projecting studs on the triangular blocks fit into the slots in plate 22 is shown, a ball bearing roller sleeve being revoluble around the body of each stud. The rollers are regarded as essentially stud elements. Spacer plates 39 and 40 which are part of the structure of tube 34 provide support for collar plate 38 and complete the enclosure of the cavity volume. In this way the cavity is completely defined by a conducting surface. The spacer plates are joined by a piece of glass or some other suitable non-conducting material, the volume thus formed being sealed to allow evacuation of tube 34.

A gear segment .3 is attached concentrically to plate 22 by screws 44, and engages with worm gear 45 which is mounted on shaft 45, an extension of which (not shown) may be brought to any convenient place to allow ready rotation of worm gear 45. Since the cavity volume and hence the resonant frequency of the cavity will be substantially proportional to the amount of rotation of shaft 46, any indicating means (not shown) calibrated directly in terms of frequency may be provided in conjunction with shaft 46.

In turning the cavity to a desired frequency, shaft 46 is rotated, worm gear 45 turning with it. Gear segment 43 being rigidly fixed to plate 22, the two rotate together. The studs 26 riding in the slots 23 in plate 22 are cammed inward by the curved sides of the slots under clockwise rotation of plate 22 relative to the ring 4. As shown in Fig. 1, each block 5 overlaps the precedingone and as the blocks are forced inward the sides of the hexagon are shortened with a resulting reduction in cavity volume.

It is to be emphasized that what has been described hereinbefore is only a preferred embodiment of the present invention and modifications and adaptations may be made without exercise of inventive ingenuity. Hence all these and such further variations are claimed as may fall fairly within the spirit and scope of the hereinafter appended claims. I

Thus, while the specific embodiment shows the formation of a hexagonal chamber, the number of wall-forming members 5 may be decreased and reshaped to form another shape of chamber, or may be increased so as to form a greater number of symmetrically arranged peripheral wall elements.

I claim:

1. A cavity resonator having a chamber comprising a plurality of movably mounted perimetral chamber-bounding members symmetrically positioned in a common plane and shaped so that an inner face of each of said chamber-bounding members taken together with similar faces of the other chamber-bounding members forms a substantially regular and continuous geometrical figure symmetrically around a common center, guide means for the last-named members to guide them in paths divergent from respective radii of said center, said guide means and chamberbounding members being constructed for translative movement of the latter longitudinally of said guide means relatively to said center, including means to fix the chamber-bounding members in orientation relative to respective radii from said center through the chamber-bounding members throughout said translative movement, and common operating means engaged with said chamber-bounding members to move all of the latter translatively and simultaneously along said paths so that on inward movement of the chamber-bounding members they will converge toward said center while retaining the same continuous relation to said geometrical figure, for altering the tuning of said resonator.

2. The structure of claim 1 in which respective springs are coengaged between each two mutually adjacent wall members tending to move them relatively along respective said paths in a direction toward one limit of their adjustment with respect to said center, yieldable to said operating means, whereby loose movement of the wall members is prevented and stability of tuning enhanced.

3. The structure of claim 1 in which each wall member laps said inner-face of the next adjacent wall member in one direction so as to slide thereupon under operation of said operating means, and respective springs coengaged between each two mutually adjacent wall members under stress tending to move them relatively, and reacting generally in a direction parallel to said inner-face of the wall member and tangential to a point on the wall member outwardly of the line of force exerted by said spring thereon, whereby to develop a moment acting to press the extreme lapped portion of each wall member against said inner-face of the next adjacent wall member. 7

4. A variable-cavity resonator comprising parallel walls, a frame bounding a space therebetween, a plurality of chamber-bounding wall members within the frame movable inwardly and outwardly in a plane between said parallel walls and fitted to the parallel walls, said wall members having outer extreme positions with inner faces normal to respective radii of the center of a chamber space within said wall members, guides engaged with respective said wall members and arranged on lines diagonal to said respective radii of the guided wall members, each said Wall member having a lateral face parallel to and fitted slidingly to said inner face of the wall member next thereadjacent in one direction, and means to move all said wall members transtween, a plurality of chamber-bounding wall members within the frame movable inwardly and outwardly in a plane between said parallel walls and fitted to the parallel walls, said wall members having outer extreme positions with inner faces normal to respective radii of the center of a chamber space within said wall members, each said wall member having an inner base face and one side corresponding to the base and one side of a constructive isosceles triangle, having its apex adjacent the said frame, said inner face being symmetrically normal to a respective in tersecting radius of said center, and said side of each wall member being aligned with the base of the next wall member in one direction so as to slide thereon, guide members fixed on the frame engaged with respective wall members and extending parallel to the geometrically projected second side of said constructive triangle of the next adjacent wall member whereby all said wall members may slide on said guides with their said sides slidingly engaged against base faces of the next adjacent wall members, and means to move all the wall members simultaneously and translatively in similar generally radial directions on the guides. l

6. The structure of claim 4 in which said guide means for each wall member is a rectilinear pin having an end connected to the frame and projected inwardly, each wall member having a part slidingly receiving the respective pin.

'7. The structure of claim 5 in which a protractile spring is confined between mutually adjacent parts of each two wall members reacting generally parallel to said side of one wall member of the two whereby to tend to rotate at least one in a direction to press mutually lapped side and base surfaces of the two wall members into good bearing.

EDWARD G. MARTIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date I 532,327 Levy Jan. '8, 1895 1,288,555 Fritz Dec. 24, 1918 2,044,413 Weyrich June 16, 1936 2,206,683 Wolff July 2, 1940 2,323,201 Carter June 29, 1943 2,410,109 Schelleng Oct. 29, 1946 2,487,619 Usselman Nov. 8, 1949 

